Idiopathic retroperitoneal fibrosis (IRF) is a rare disease characterised by the presence of a retroperitoneal periaortic fibro-inflammatory tissue which may entrap the ureters and cause renal failure. The treatment of IRF is not well established. Corticosteroids are frequently used, but the anti-estrogen agent tamoxifen has also been reported to be effective in a number of reports. However, no randomised trials have been published so far. The aim of the present study is to compare the efficacy of prednisone and tamoxifen in the treatment of IRF.